1. Carlyle/Riverstone Global Energy and Power Fund III, L.P. ("C/R Fund III"), C/R Energy III Cobalt Partnership, L.P. ("C/R III Cobalt"), Riverstone Energy Coinvestment III, L.P. ("REC III") and Carlyle Energy Coinvestment III, L.P. ("CEC III") and the record holders of 20,222,488, 8,712,859, 828,383 and 179,993 shares of common stock, respectively, following the transactions reported herein. C/R Energy GP III, LLC exercises investment discretion and control over the shares held by each of C/R Fund III and C/R III Cobalt through their mutual general partner, Carlyle/Riverstone Energy Partners III, L.P., of which C/R Energy GP III, LLC is the sole general partner. C/R Energy GP III, LLC has the power to direct the voting and disposition of the shares held by each of REC III and CEC III.

2. Riverstone Coinvestment III GP, L.L.C. is the general partner of REC III. Riverstone Holdings LLC is the managing member of Riverstone Coinvestment III GP, L.L.C. Pierre F. Lapeyre, Jr. and David M. Leuschen are the managing members of Riverstone Holdings LLC and, in such capacity, may be deemed to share beneficial ownership of common stock beneficially owned by Riverstone Holdings LLC. Such individuals expressly disclaim any such beneficial ownership. Carlyle Energy Coinvestment III GP, L.L.C. is a subsidiary of TCG Holdings, L.L.C., which exercises investment discretion and control over the shares held by CEC III, subject to contractual commitments that CEC III invest and divest side-by-side with C/R III Cobalt.

3. C/R Cobalt Investment Partnership, L.P. ("C/R Cobalt") and C/R Energy Coinvestment II, L.P. ("C/R Coinvest") are the record holders of 18,257,252 and 1,705,231 shares of common stock, respectively, following the transactions reported herein. C/R Energy GP II, LLC exercises investment discretion and control over the shares held by each of C/R Cobalt and C/R Coinvest through their mutual general partner, Carlyle/Riverstone Energy Partners II, L.P., of which C/R Energy GP II, LLC is the sole general partner.